Crayfish perform housekeeping duties the benefit of which I think contribute in ways beyond their role as prey. Their activity helps to recycle nutrients and maintain a healthy pond bottom. This activity has the potential of increasing the production of other prey organisms. IOWs having a well balance crayfish population could make more prey of other species available too. This effect has also been noted of TP which perform similar duties but crayfish can perform these tasks and reach high standing weights when pond temps are cool before these other species really get active.
To be sure, too many can cause unwanted side effects like water that is too turbid but I think in a good balance this turbidity would show up in mid to late spring when numbers are peaked and then diminish significantly the remainder of the year. I am planning for them in my pond when that happens and I will also likely culture them as well. They are important in your water or they wouldn't be so prevalently consumed.
